I understand that the imidazole ring in histidine is aromatic. I also realize that it retains it's aromaticity when protonated.
I am wondering why it is not mentioned at all in basic text books such ...

I'm looking at this diagram of homocysteine metabolism and see two distinct pathways that the amino acid may get metabolized to: with vitamin B12 it gets converted back into methionine, while with B6 ...

Amino acids with non‐ionizable side chains are zwitterions when they are at physiological pH, pH 7.4. This is what my book says. But I do not understand why. The Pka for a carboxyl group is around 3 ...